Coffee Data Science
Rethinking Refractometers: VST, Atago, and DiFluid; Part 3
By: Robert, Joe, and Jeremy
We have one more chunk of data from another set of DFT devices. We previously looked at two other batches of devices and experiments. Those tests highlighted the difficulty of understanding a refractometer’s performance using a single device, as well as the performance across devices.
The other challenge becoming more evident is the difficulty in measuring performance even for well known and calibrated devices like VST and Atago. This doesn’t mean refractometers aren’t very useful in coffee, but we need to have a better understanding about what they are reading and how Total Dissolved Solids (TDS) content is extrapolated from that reading.
The Data
The data was collected in three batches across 16 DFT devices, 1 VST, 1 Atago Coffee, and 1 Atago RX-5000i. Each batch used a different set of DFT devices (5, 6, and 5 respectively). Additionally, some samples were analyzed with a moisture balance, providing a groundtruth measurement. Several solutions were used, each providing different insight:
- Sucrose solution (the basis for Brix measurement; well-established normative data; a “clean” assessment of hardware)
- Instant Coffee at Espresso Strength (high coffee soluble concentration with minimal interference from non-solubles; increased difficulty from sucrose, requiring software conversion of refractive index reading to coffee solubles)
- Instant Coffee at Filter Strength (low coffee soluble concentration with minimal interference from non-solubles; reduced signal strength compared to instant coffee espresso but relatively low noise compared to real-world solutions as instant coffee is almost entirely coffee solubles — 99.9%
- Espresso (real-world application at high coffee soluble concentration; a difficult testing solution with increased noise but strong signal)
- Filter Coffee (real-world application at low coffee soluble concentration; the most difficult testing solution with decreased signal and increased noise, testing robustness of both hardware and software)
It should be noted that not all test sets used all of these solutions. In testing, some samples were filtered with a syringe filter and some were not, but it is explicitly stated in the charts.
In this article, we will look at all four solutions: two at espresso strength, two at filter strength.
Data
Each sample had three readings of the same solution. The Atago Coffee, VST, and Atago Alpha trend very well together, but as previous experiments showed, sucrose has a much cleaner effect on refractive index. Similar to other experiments, one could get a device like the DFT D, where its accuracy is very close to VST/Atago.
The more challenging scenario is filter strength coffee. There is also a bit of variation in VST, Atago Alpha, and Atago Coffee. The DFT devices in the instant coffee experiment were closer to the groundtruth, which is curious, though the number of samples is too small to apply any statistical analyses.
For instant coffee, all the samples are far from groundtruth, but they are close to each other. This likely suggests the conversion algorithms used in the VST and Atago Coffee attempt to account for some signal noise (Atago Alpha’s readings were converted to TDS using a formula derived from VST & Atago Coffee readings, so tracking with the coffee-correlated refractometers is to be expected).
These refractometer results point to some fundamental gap in how coffee affects refractive index. Part of this gap could be as simple as calibration trouble, but in these experiments, great care was taken to calibrate all the devices at the start of the tests. This data also points towards the need for multiple hardware samples to truly evaluate the usefulness of any given refractometer.
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